1. Field of the Invention
The present invention relates to a method for applying a conductive paste to electronic parts and a device for applying conductive paste used in the method.
2. Description of the Related Art
In a fabrication process of an electronic component, such as a laminated ceramic chip capacitor for example, a method has been widely used in which a conductive paste is applied to the electronic parts and is then cured in order to form external electrodes on electronic parts.
To apply the conductive paste for external electrode formation to the electronic parts, a tabular holding jig 51, shown in FIG. 1, and a guide plate 61, shown in FIGS. 2 and 11, are typically used to cause conductive paste 60 to stick to electronic parts 55, as shown in FIG. 4. As shown in FIG. 1, the holding jig 51 is made from an elastic material (such as rubber) 57, which covers metal core materials 56, shown in FIGS. 2 and 3, and has a plurality of holding holes 52 whose side wall is composed of the elastic material (rubber) 57. On the periphery of the holding jig 51, a reinforcing metal frame 58 is attached. As shown in FIGS. 2-4, the holding jig 51 holds electronic parts 55 by using a guide plate 61, and then dips ends of the electronic parts 55 protruding from holding holes 52 of the holding jig 51 into the conductive paste 60 formed on a surface plate 54.
In this method, as shown in FIGS. 1 to 3, the guide plate 61 is aligned with the holding jig 51 such that the center axes of the through-holes 62 are respectively aligned with the center axes of the holding holes 52. The through-holes 62 of the guide plate 61 accommodate the electronic parts 55. With center axes of a plurality of press pins P aligned with the center axes of the holding holes 52, these press pins P simultaneously push the electronic parts 55 held in the through-holes 62 of the guide plate 61 into the holding holes 52 of the holding jig 51, one side of which is in contact with a base plate 53 covering the entire area defined by the outer periphery of frame 58. After the holding jig 51 holds the electronic parts 55, the conductive paste 60 held on the surface plate 54 sticks to the ends of the electronic parts 55 which protrude from the holding holes 52 of the holding jig 51, as shown in FIG. 4.
However, this method causes elastic deformation of the elastic material (rubber) 57 of the holding jig 51 due to pushing pressure when the plurality of press pins P push the electronic parts 55 into the holding holes 52 of the holding jig 51. In particular, the center portion of the holding jig 51 is compressed and, thus, the holding holes 52 generate an inward pressing force. Therefore, the electronic parts 55 are not inserted deeply. In addition, since the entire holding jig 51 is deformed to be convex upward, the protrusion distances (protrusion heights) of the electronic parts 55 from the holding holes 52 in the center portion disadvantageously become longer.
With such unequal protrusion distances, the electronic parts 55 having a long protrusion distance press the conductive paste 60 more strongly that those having a short protrusion distance when the electronic parts 55 are dipped in the conductive paste 60 on the surface plate 54, thus strongly pushing the conductive paste 60 in the transverse direction. Accordingly, when the electronic parts 55 are pulled up from the conductive paste 60, only a small amount of the conductive paste 60 flows in between the electronic parts 55 and the surface plate 54, and therefore, the thicknesses of the conductive paste 60 applied to the ends of the electronic parts 55 disadvantageously become thinner.
To solve these problems, as shown in FIG. 5, the guide plate 61 is aligned with the holding jig 51 underneath the guide plate 61 such that the center axes of the through-holes 62 of the guide plate 61 are aligned with the center axes of holding holes 52 of the holding jig 51. The through-holes 62 accommodate the electronic parts 55. With the center axis of a press pin P aligned with the center axis of the holding hole 52, the press pin P pushes up one of the electronic parts 55 held in the corresponding through-hole 62 into the holding hole 52. Upon completion of the insertion, the press pin P moves to the center of the adjacent holding hole 52 (i.e. the center of the adjacent through-hole 62) and pushes up another electronic part 55 in the same manner. This operation is repeated until all the electronic parts 55 are inserted into the holding holes 52 (refer to, for example, Japanese Unexamined Patent Application Publication No. 9-22854).
Since the insertions of the electronic parts 55 into the holding holes 52 of the holding jig 51 are performed for each holding hole one at a time in this method, the holding jig 51 receives a smaller stress compared to the known method in which all the insertions into the holding holes 52 are simultaneously performed. Accordingly, curvature deformation of the holding jig 51 when holding the electronic parts 55 can be decreased.
However, this method increases the time for holding a large number of electronic parts and, thus, the productivity is disadvantageously decreased.
Another method for a holding jig to hold electronic parts has been proposed in which the conductive paste can be uniformly applied to the electronic parts even when the protrusion distances of the electronic parts from the holding jig are unequal (refer to, for example, Japanese Unexamined Patent Application Publication No. 5-182879).
In this method, as shown in FIGS. 6A to 6D, when ends of the electronic parts 55 are held by the holding holes 52 of the holding jig 51 and the protrusion distances of the electronic parts 55 from the holding holes 52 are unequal, as shown in FIG. 6A, the other ends of the electronic parts 55 protruding from the holding holes 52 are urged onto the surface plate 54, with the holding jig 51 parallel to and distant from a flat surface of the surface plate 54 as shown in FIG. 6B, to align the protrusion distances. That is, since parts of the elastic material 57 around the holding holes 52 simultaneously receive a uniform elastic strain, the protrusion distances become substantially equal and, thus, vertical positions of the electronic parts 55 are aligned, as shown in FIG. 6B.
Thereafter, as shown in FIG. 6D, the ends of the electronic parts 55 protruding from the holding holes 52 of the holding jig 51 are dipped into the conductive paste 60, shown in FIG. 6C, which is developed in a predetermined thickness on the surface plate 54. Thus, the conductive paste 60 is applied to the ends of the electronic parts 55.
However, since the ends of the electronic parts 55 protruding from the holding holes 52 are urged onto the surface plate 54 with the holding jig 51 parallel to a flat surface of the surface plate 54 as shown in FIG. 6B to align the protrusion distances in this method, an extra step is required and, therefore, the fabrication process is disadvantageously complicated and costly.
Also, since this method still causes the elastic deformation of the elastic material (rubber) 57, the protrusion distances cannot be exactly equal, which is a problem.